Jadzia Dax, without question. I even named one of my cats after her! Well... and another after 7of9 (name was Seven, without the "of nine").
Some close runners up would be:
Robin Lefler -- Ashley Judd (*drool*)
Deanna Troi -- Marina Sirtis
B'lanna Torres-Paris -- Roxanne Dawson (heh, even _with_ the 1/2 Klingon makeup. Nothin' wrong with datin' outside your species... nothin' at all...)
Ezri Dax -- Nicole DeBoer
Honorable mention:
7 of 9 -- Jeri Ryan (Easy on the eyes, but a little too Barbie-Doll like to win my heart.)
Leeta -- Chase Masterson (A little bit too much ?topheavy? for my likes.)
T'Pol -- Jolene Blalock (Vulcan == Sexy. Now, if they could just loosen up a little bit.)